While very interesting, this is not quite as conclusive as the article suggests. The overall reflectivity of Phobos is much lower than of Mars rocks. Carbonaceous chondrites are the best match. Also, it is expected that there is a good collection of rocks blasted off of Mars that have landed on Phobos (hence a sample return mission from Phobos is expected to bring back Mars rocks as well as whatever Phobos is made of). The Mars-like minerals could be from these rocks, not what the bulk of Phobos is made of.

"The overall reflectivity of Phobos is much lower than of Mars rocks."

Well, the albedo of Phobos is lower than the albedo of Mars, but most of Mars is covered with bright dust. I'm not so sure that Phobos is darker than actual Mars rocks, most of which would be of basalt-like composition. I thought the story was a bit misleading in another way... the rejection of a carbonaceous asteroid composition goes back quite a long way, based on things like the Mars Pathfinder multispectral imaging of Phobos, and Phobos-2 spectral data. Several papers have already said Phobos is not asteroid-like in composition.

Phyllosilicates are very popular these days. Last time I was at JPL there was a disheveled looking guy leaning on his car a few blocks down the street. As I walked past he said "Pssst, hey buddy, you look like a scientist. You want to see some phyllosilicates?" He popped open the trunk of his car and said, "C'mere man check these out. I got serpentine, micas, clays, talc, whatever you want, real cheap."

I had a thought today about a possible method of estimating the age of Phobos' formation. Since Phobos is slowly spiraling in towards Mars (and is estimated to break up in 30-50 million years), could not a calculation be done in 'reverse' to see how long ago Phobos would have been in/near a areo-synchronous orbit around Mars? In other words, we know that Phobos must have been formed/captured somewhere inside a synchronous orbit, otherwise, it would never have spiraled in-ward in the first place. Therefore running the calculation backward would give a maximum age in which Phobos would have formed or captured.

Right now, Phobos orbits approximately 6,000k above Mars and the areo-synchronous altitude is 17,200km, a distance of 11,200km. I understand that Phobos is descending at a current rate of ~1.8cm per year. That rate of orbital decay is not constant (it increases the closer it descends), but I am sure someone has an equation (differential formula?) which could calculate that.

Running time backwards we see Phobos spiraling back out, but as it does so it crosses various resonances with Deimos, and Deimos might be subject to forces changing it's orbit too. I'd think it would get difficult to know how long a particular resonance might have persisted, and which ones might have been active.

The problem is that simply reversing the orbit dynamics in time gets to the point where you require another body in the equation to effect the orbital capture. And it's impossible to tell what those dynamics were.

Myself, I prefer the theory that a fairly large and rapidly spinning body broke up when it passed within Mars' Roche limit. Part of of it impacted Mars, part of it achieved escape velocity, and two pretty big chunks ended up in stable orbits. Other chunks ended up in unstable orbits and eventually hit Mars.

At what time this happened is probably most easily constrained by looking at the age of craters/basins that could have been caused by such a catastrophic impact. You can place the event at almost any point in the time-reversed orbital history of the moons by simply adjusting the size, speed and trajectory of the body that came apart, so such a reverse-time orbital analysis would be less useful for constraining the timeframe, I would imagine.

-the other Doug

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“The trouble ain't that there is too many fools, but that the lightning ain't distributed right.” -Mark Twain

Running time backwards we see Phobos spiraling back out, but as it does so it crosses various resonances with Deimos, and Deimos might be subject to forces changing it's orbit too.

Good point which I did not consider. But would resonances be significant for such low mass moons? Unlike the Galilean moons, which have considerable mass and are in resonance, these moons are only 22km (Phobos) and 6km across (Deimos). Even if we put Phobos near the areo-synchronous altitude of 17,000km, it would still be ~6,000km from Deimos. I would think gravitational disturbances with such low masses would be very minor at that distance...

Even if there were some sort of resonance, would it prevent the kind of tidal orbital decay of a moon under the areo-synchronous altitude?

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